Manufacturers label wood screws with two numbers to indicate their size: the gauge or "number" and the length in inches. You may also see a third number, the threads per inch. If so, it's hyphenated after the gauge. For example, a 7 x 2" screw has a gauge of 7 and is 2 inches long. A 7-15 x 2" designation means the screw is gauge 7, has 15 threads to the inch and is 2 inches long. The gauge depends on the diameter of the shank, which is the part of the screw below the head but before the threads begin.
Place the shank of the screw between the jaws of calipers that can measure to thousandths of an inch. Adjust the calipers to read the diameter in inches, expressed to three decimal places, such as .177 inches.
Subtract .06 from the measurement.
Divide the result by 13 and multiply by 1,000 to find the gauge of the screw. For example, .177 minus .06 is .117. Divide .117 by 13 to get .009, then multiply by 1,000 to calculate that you've measured a screw with a gauge number of 9.
If you don't have calipers, estimate the gauge by holding a ruler behind the shank and measuring the diameter in 64ths of an inch. Use a screw gauge chart to convert the diameter to a gauge size, though the difference in diameter between gauges is so small that you may not be able to measure precisely enough to tell the gauge exactly.